1. Field of the Invention
The present invention relates to a structure and manufacturing method of a TFT type liquid crystal display, and particularly, to a TFT designed to prevent a drain electrode from being disconnected from a pixel electrode.
2. Description of Related Art
In general, a TFT type LCD comprises a bottom plate, an upper plate and a liquid crystal which is filled in between the two plates. A polarizing plate which polarizes a visible ray so that only one aspect of the ray could be seen is attached at both sides of the two plates. A polarizing plate is attached at one side, and a color filter and a common electrode are formed on the other side of the upper plate. A plurality of gate bus lines 10 and a data bus line 20 are in a matrix form and on a certain space formed by the crossing of a plurality of gate bus lines 10 and a data bus line. A pixel electrode 40 is located in the structure of the bottom plate. Structure of the bottom plate of the active matrix type LCD is illustrated hereafter with reference to FIGS. 1a and 1b.
A plurality of gate bus lines 10 is formed horizontally on the bottom plate of the LCD, and a gate electrode 11 is formed by deriving from the gate bus line 10. A plurality of data bus lines 20 is formed perpendicularly to each of the gate bus line 10, and a source electrode 21 is formed by deriving from the data bus line 20. The switching element of the TFT structure comprises a gate electrode 11, a source electrode 21 and a drain electrode 31 by forming the drain electrode 31 opposite to the source electrode 21. The manufacturing step of the bottom plate of the active matrix type LCD is illustrated below with reference to FIGS. 2a to 2f.
Bottom plate or substrate 1 is formed with a glass substrate. A gate electrode 11, which is derived from the gate bus line, is formed through patterning on the bottom plate 1. A gate anodizing layer 12 is formed on the gate electrode 11. A gate insulating layer 13 is deposited on the bottom plate 1 where the gate anodizing layer 13 is formed. A gate insulating layer 13 is then formed with SiNx, SiOx and the like which have good interfacial property with a-Si, good adhesiveness with the gate electrode 11 and the bottom plate 1 and a high insulating characteristic. A semiconductor layer 14 is formed by depositing a-Si which has a high moving density of carrier on the gate insulating layer 13. An ohmic contact layer 15 is formed on the semiconductor layer 14 for satisfactory ohmic contact between source electrode 21 and drain electrode 31. A Ti metal layer is applied on the entire surface of the bottom plate 1 by a sputtering method and then the data bus line 20 which acts as a signal line, source electrode 21 which is derived from data bus line 20 and drain electrode 31 which acts as an output terminal located opposite to the source electrode 21 are formed by patterning the Ti metal layer. The switching element of TFT structure comprising gate electrode 11, semiconductor layer 14, source electrode 21 and drain electrode 31 is formed through the sequential steps mentioned above.
A protection layer 23 is formed by applying an organic insulating layer, such as BCB and the like, on the switching element. Referring to FIG. 2a, photo resister 60 is developed into a certain pattern by exposing with mask after applying photo resister 60 on the protection layer 23 using a spin coating method.
The bottom plate 1, including photo resister 60 which is developed into a certain pattern, is etched by plasma gas like SF.sub.6 /O.sub.2 or CF.sub.4 /O.sub.2 inside the etching chamber and, through this process, a part of the protection layer 23 is eliminated. The photo resister 60 on the protection layer 23 is concurrently eliminated by ashing with 0.sub.2 gas. Therefore, a contact hole 50, which is exposed through the place where drain electrode 31 and protection layer 23, are partly eliminated as shown in FIG. 2b.
An ITO layer 42 is deposited on the entire surface of the protection layer 23 where the contact hole is formed by a sputtering method as shown in FIG. 2c.
Another photo resister 60 is applied on the ITO layer 42 by a spin coating method. By exposing the spin coated photo resister 60 using a mask, it is developed into a certain pattern as shown in FIG. 2d.
The pixel electrode 40 is then formed by etching the bottom plate 1 which includes photo resister 60 developed into a certain pattern with etching solution like HCI. The photo resister 60 remaining on the pixel electrode 40 is eliminated by using an organic solution which is a mixture of NMP{N-Methyl-Pyrrolidone), alcohol and amine. The pixel electrode 40 which is in contact with drain electrode 31 through contact hole 50 is formed as a result as shown in FIG. 2e.
An orientation layer with a polyimide layer and the like is applied on the entire surface of the bottom plate of the LCD which includes a plurality of switching elements and pixel electrodes 40 that are formed through sequential steps illustrated in FIGS. 2a-2f and described above. The manufacturing step of the bottom plate 1 of the LCD is carried out by using a baking method which is a heat treatment of high temperature approximately 150.about.200.degree. C. at the bottom plate of the LCD where the orientation layer is applied.